1. Field of the Invention
This invention relates to data transmission and more particularly to on-die termination associated therewith.
2. Description of the Related Art
High-speed serial data transmission systems are being adopted to overcome limitations of data transmission rates of conventional parallel data transmission systems. Typically, a serial data transmission system utilizes a differential signal structure to increase noise immunity. In such a serial data interface, impedance calibration between a transmitter and a receiver is important. Without proper impedance matching between a transmitter and a receiver, levels of transmission and receiving signals can be distorted due to reflected waves, and as a result, the bit error rate of data transmission increases.
As the performance requirements of serial data transmission systems continue to increase, there are increasing requirements to deliver signal integrity that enables reliable operation at higher signaling rates. Signal line terminations are useful elements in the management of signal integrity and may be used external to the device or within the device itself. Incorporating a resistive termination within the device, which is often referred to as On-Die Termination (ODT), improves the signaling environment by reducing the electrical discontinuities introduced with off-die termination. Variations in the fabrication process as well as fluctuations in voltage and temperature result in variability in the resistive characteristics of the ODT elements.
FIG. 1 illustrates a block diagram of one approach for ODT calibration for pull-up termination. In the illustrated embodiment, ODT calibration block 100 includes the array of switches (S1-Sn) and resistors (R1-Rn), which are controlled by the feedback loop that includes N-bit decoder 101, up/down counter 103, comparator 105 and voltage reference 107. The N-bit decoder 101 generates the thermometer digital code (DCNTL) from the binary N-bit output of the up/down counter 103 to set the resistance of the ODT array 111. Initially the N-bit output of the up/down counter is set to any value and increases or decreases depending on the value of COUT. Table 1 shows a truth table of an exemplary thermometer decoder. DCNTL[1:n] corresponds to S[1:n], which is a switch array. DCNTL[x]=0 and 1 mean Sx turns off and on, respectively. As the N-bit output of the up/down counter increases in value, the number of switches that are turned on increases. As the number of output bits increases, the number of switches that are controlled increases. For example, a 3-bit thermometer decoder generates 8 switch signals while a 5-bit thermometer decoder can generate signals for a 32 switch array.
TABLE 1N-bit (decimal)DCNTL[1]DCNTL[2]DCNTL[3]. . .DCNTL[2N−2]DCNTL[2N−1]DCNTL[2N]010000001110000021110000. . .. . .. . .. . .. . .. . .. . .. . .2N − 311111002N − 211111102N − 11111111
FIG. 2 illustrates an ODT transfer curve, which shows the relationship of ODT resistance (RODT) and DCNTL. As the DCNTL value increases, RODT decreases because of the parallel configuration of the resistors. RODT together with REX 109 (an external precision off-chip resistor on the printed circuit board (PCB)), determines VCAL. VCAL=VDD*REX/(RODT+REX). If VREF can be generated accurately, then accurate RODT can be approximated as REX*(VDD/VREF−1) because VCAL approaches VREF through the feedback loop.
The comparison between VCAL and VREF is made in the comparator 105. The following describes the transfer function of comparator.
IF VCAL>VREF THEN COUT=‘high’                ELSE COUT=‘low’        
VREF is generated from voltage reference 107, which is a voltage divider using resistor string. FIG. 3 illustrates an example of a voltage reference. As long as the ratio of RA and RB in the voltage divider is accurate, the accuracy of RODT depends on that of the external resistor off-chip REX. Typically, the accuracy of REX is at least 5%.
The up/down counter 103 counts up or down depending on the COUT state. The logic value ‘high’ and ‘low’ of COUT makes the up/down counter count downward and upward, respectively. While VCAL<VREF, COUT stays ‘low’ and the up/down counter counts upward. On the other hand, while VCAL≧VREF, COUT stays ‘high’ and the up/down counter counts down. Therefore, once the state of COUT is changed, the up/down counter counts in the opposite direction at the next feedback operation and COUT toggles. FIGS. 4a and 4b illustrate the calibration operation in the time domain. FIGS. 4a and 4b illustrate embodiments in which Rinitial (the initial value of RODT) is lower and higher than Rtarget, respectively, where Rtarget=REX*(VDD/VREF−1). If the state of COUT is not changed until the up/down counter counts down to 0 or up to 2N−1, it means that RODT is not able to achieve the target impedance with the proper accuracy.
Once RODT reaches Rtarget, at time T1 it toggles between (Rtarget−Δ1) and (Rtarget+Δ2) because COUT also toggles and the up/down counter counts in the opposite direction at the next feedback operation when the state of COUT is changed. The value of (Δ2−Δ1) is defined as the resolution of RODT and is determined by the number of array switches (S1-Sn) and resistors (R1-Rn). It means that the N-bit output of the up/down counter determines the resolution. As the number of bits (N) of the N-bit output increases, one can achieve a higher resolution of RODT for calibration, but that requires more switches and resistive components. Thus, there is trade-off between resolution and area.
FIG. 5 depicts the block diagram of a differential TX (transmitter) with an impedance calibrated pull-up termination. Actual calibration occurs using ODT 501, which is coupled to REX and a feedback loop. Identical ODTs 503 and 505 (or at least substantially identical, given possible process variations and temperature gradients) are used for the pull-up terminations at the transmission nodes 507 and 509 at the output of the transmitter 511. DCNTL is generated through the calibration feedback loop and feeds the switch arrays on the TX pull-up terminations.
FIG. 6 shows the application impedance calibrated pull-up termination for a receiver (RX). The same calibration concept as shown in FIG. 5 can be used at the transmission nodes 601 and 603 at the input of the receiver RX 605.
The same idea can be used for the impedance calibrated pull-down termination. FIG. 7 and FIG. 8 show TX and RX with impedance calibrated pull-down termination, respectively. The differences from pull-up termination configuration are REX is connected to VDD and switches are connected to GND.